The present invention relates generally to the field of semiconductor fabrication and, more particularly, to a body contact silicon-on-insulator transistor and method.
Because of fundamental limitations on bulk insulated gate field defect transistors (MOSFETS), operational improvement of present devices generally requires significantly changing the basic operation of the transistor. One important area of development over the last several years has been the use of silicon-on-insulator (SOI) transistors to improve performance of MOSFETS.
SOI transistors generally include a silicon semiconductor substrate deposited over an insulating layer. SOI transistors also generally include an electrically isolated body formed in the silicon semiconductor substrate and disposed beneath a gate region of the transistor. Thus, SOI transistors generally require a body contact for coupling the body of the SOI transistor to a fixed potential. For example, SOI transistors generally require connecting a p-type conductivity body, in the case of an n-channel MOSFET, or an n-type conductivity body, in the case of a p-channel MOSFET, to a fixed potential.
One method of connecting the body of the SOI transistor to a potential is to provide an edge contact to the body. Proving an edge contact to the body generally requires a T-shaped or H-shaped gate poly layout. However, T-shaped and H-shaped gate poly layouts generally add significant parasitic capacitance to the SOI transistor due to the portion of the poly layer not contributing to drive current of the transistor, especially if the gate electrode properties are generally uniform.
Accordingly, a need has arisen for an improved method for manufacturing a body contact silicon-on-insulator transistor. The present invention provides a method for manufacturing a body contact silicon-on-insulator transistor that addresses shortcomings of prior methods and transistors.
According to one embodiment of the present invention, a method for fabricating a body contact transistor includes providing a semiconductor substrate on an insulator and lightly doping the semiconductor substrate to form a body region. The method also includes forming a gate on an upwardly facing surface of the semiconductor substrate and separated from the upwardly facing surface of the semiconductor substrate by a gate insulator layer. The gate defines a source region, a drain region, and a contact region. The method further includes masking a portion of the gate and the contact region and heavily doping the source region, the drain region, and an unmasked portion of the gate with a material having a conductivity substantially opposite a conductivity of the body region. The method further includes heavily doping the contact region with a material having a conductivity substantially opposite to the conductivity of the source region and the drain region.
According to another embodiment of the present invention, a body contact transistor includes a body region of a first conductivity type formed over an insulator. The transistor also includes a source region and a drain region, each having a second conductivity type substantially opposite the first conductivity type. The source and drain regions are formed adjacent to the body region. The transistor further includes a gate formed over the body region and separated from the body region by a gate insulator layer. The gate includes a doped portion of a third conductivity type disposed adjacent the source region and the drain region. The third conductivity type is substantially opposite to the first conductivity type. The gate also includes an undoped portion disposed at an end of the gate and adjacent a contact region. The contact region is coupled to the body region.
Technical advantages of the present invention include substantially reducing the parasitic capacitance of a silicon-on-insulator transistor. For example, according to one aspect of the present invention, the gate of the transistor includes a doped portion and an undoped portion. The doped portion of the gate includes a conductivity type substantially opposite of a conductivity type of a body region of the transistor. Source and drain regions of the transistor are formed on either side of the doped portion of the gate. The undoped portion of the gate is disposed at an end of the gate and adjacent a contact region of the body. The undoped portion of the gate is electrically coupled to the doped portion of the gate through a silicide layer formed on an upwardly facing surface of the of the gate. Because of the creation of a depletion region in a portion of the gate proximate the gate insulator layer, the effective gate insulator thickness is increased, thereby substantially reducing the parasitic capacitance between the gate and the body region in the undoped portion region of the gate.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.